Freiberger Drum Sextant (Trommelsextant)

10 08 2011

This post is preceded by one on the Freiberger Yacht sextant and two on the Freiberger Skalensextant

Readers looking for a manual that helps with maintenance and repair of the Freiberger Trommelsextant (drum sextant) will find my SNO-T Sextant Manual very useful, as the design of the one is based on the other. While the manual describes the SNO-T, it also gives an account of the Freiberger drum sextant where its design details differ. See under “The USSR SNO-T sextant” and “Buy”

The firm of Freiberger Präzisionsmechanik has been in existence since  late in the eighteenth century and by the 1870s had a large workshop employing over eighty people in the manufacture of surveying instruments. At the end of WW II it was overrun by  Soviet forces and dismantled, leaving only fifteen workers to carry out maintenance on surveying equipment. It was refounded in 1950 and in that decade the trommel sextant was developed. As far as I know, the firm had never previously made sextants.

The sextant is unusual in several respects. While the over-all shape and placement of shades and mirrors is conventional, it has a die-cast aluminium alloy frame, which combines lightness with a strength and hardness near to that of mild steel. The ladder or three circle patterns of its main competitors were ignored. Material is concentrated around the edges and the whole stiffened by a central web (Figure 1). The worm runs in a rack machined directly into the edge of the limb, thus avoiding the complication of attaching a bronze rack to an aluminium frame.  The substantial but unseen bronze worm seems to run very well against the alloy rack.

Figure 1 : Freiberger drum sextant, front view.

The very substantial index arm lies behind the web of the frame and is bridged by a casting to which is attached the handle (Figure 2). The upper end of the index arm is screwed to a large diameter bronze journal (the part that rotates in a bearing) that rotates in a bearing machined directly in the frame, thus abandoning the narrow, tapered journal and bearing in use since the third quarter of the eighteenth century. C Plath later dallied with such a bearing in their bronze-framed instruments, but they soon reverted to the tapered form.


Figure 2 : Freiberger drum sextant, rear view.

 The micrometer mechanism is concealed within an alloy casting attached to the lower end of the index arm. The cylindrical worm runs in eccentric bearings in a bronze casting that itself rotates against the force of a helical spring in bearings machined in the alloy casting. Thus, when the bronze casting is rotated, the worm swings out of engagement with the rack. The closeness of this engagement can be adjusted by means of a tangential screw whose head is just visible in Figure 2 to the left of the drum. While Freiberger chose to swing the worm out of the plane of the rack, nearly every other maker swung it out of engagement in the plane of the rack, following the pattern devised by C Plath in about 1907.  The latter method must certainly have been cheaper to manufacture, even allowing for the unecessary complexity of the worm shaft bearings in some pre-war marques. However, Freiberger’s method totally encloses the worm and solidly supports the shaft at both ends, so it is hard to imagine the shaft getting bent by an accidental knock, as had happened to at least three conventional instruments that have passed through my hands.

The sextant was usually provided with a 3½ x 40 Galilean telescope only. My own instrument has a 7 x 35 monocular, which gives a superior field of view as well as making the point of coincidence of the body with the horizon easier to determine. The vee and flat of the mounting are the reverse of all other makers, so their telescopes cannot be interchanged.

The USSR imitated Freiberger’s design in their SNO-T sextant, albeit in an instrument of slightly smaller radius and one provided with an unusually full complement of tools and spares. The edge of the SNO-T frame is  8 mm thick (compared to 3 mm), making it an even more rigid and robust instrument than the Freiberger. The bare sextants weigh 1300 and 1200 grams respectively.